Overhead projectors in a variety of styles and types are known. In one type of overhead projector the housing accomodates the light source as well as the writing plate and Fresnel lens. With this type of overhead projector the housing is block-shaped. In another type of overhead projector the light source is in the projection head. The housing then accomodates only the writing plate and Fresnel lens and has a low profile which is almost flat. The power supply of this overhead projector is either in the housing or in the support arm. In many cases the power supply is cooled by an air stream produced by a blower which likewise cools the light source and possibly the objective lens.
These overhead projectors work with a constant brightness. This has the objection that in a room flooded with daylight the brightness is frequently too limited while in a darkened room the brightness is too great. In particular during the changing of projection foils the brightness is frequently found to be too strong. It is possible to control the brightness by hand, for example by means of a simple potentiometer. However in practice such adjustment is not used for the reason that a lecturer operating the overhead projector does not wish to disturb his concentration on the lecture.
The concentration of the lecturer operating an overhead projector is also disturbed when it is necessary from time to time to adjust the picture sharpness. Moreover the ability of a lecturer to concentrate is impared by the monotonous and sleep-producing noise of the blower.